1. Field of the Invention
The present invention relates to a developing assembly that is used for an image-forming apparatus based on an electrophotographic system.
2. Description of the Related Art
A developing assembly, that does not include a developer supply member, has been proposed to downsize and reduce cost of a developing assembly. The developer supply member has a function to supply and scrape off developer (hereafter called “toner”) to/from a developing roller (developer bearing member), and is installed mainly for handling a ghost and a solid image follow-up failure. A ghost is a phenomenon that appears when a half tone image is formed after a high density solid image is formed, where the traces of the solid image appear on the half tone image. A solid image follow-up failure is a phenomenon that appears when a 100% solid image is drawn on the entire surface of an image, where the density of the rear end of the image decreases. Therefore in order to omit the developer supply member, these problems must be handled by a different means. In Japanese Patents Nos. 3272056 and 3162219, a developing assembly that does not include a developer supply member is proposed, where dielectric portions and conductor portions are regularly or irregularly mixed and distributed on the surface of a developing roller. In other words, the dielectric portion on the surface of the developing roller is rubbed by a toner layer thickness regulating member (developer regulating member) directly or via toner, whereby the dielectric portion is charged and a minute closed electric field is formed on an adjacent part with the conductor portion. The toner conveyed to the surface of the developing roller is subject to the gradient force generated by the minute closed electric field, is sucked to and borne on the surface of the developing roller.
According to Japanese Patents Nos. 3272056 and 3162219, if the charging polarity of toner is negative, the developing assembly is constructed such that (−) toner<developer regulating member<dielectric portion (+) is established in a triboelectric series. By this configuration, multilayer toner can be borne on the surface of the developer roller, and the generation of a solid image follow-up failure can be suppressed.
However the inventors discovered that if the dielectric portion is positioned to the side closer to the opposite polarity of the charging polarity of the toner than to the developer regulating member on the triboelectric series, as in the case of Japanese Patents Nos. 3272056 and 3162219, a ghost is easily generated. In the configuration of the prior art, toner borne by the dielectric portion strongly adheres to the dielectric portion electrostatically, therefore regulating the toner layer thickness is difficult, and the toner coating amount on the developing roller, when the solid white image is formed, becomes higher compared with the case of forming a solid image. This difference in the toner coating amount may appear in the image as a ghost. Further, the developer regulating member has only the function to adjust the coating amount, and has no scraping off function, hence if a low printing page is continuously outputted, toner may melt and adhere to the developing roller. Furthermore, toner may melt and adhere to the developer bearing member since the developer supply member has no scraping off function, and the dielectric portion is charged to a polarity that is opposite that of the toner. To avoid these image defects due to the melt adhesion of toner, the life of the developing assembly in some cases may be set to be short.